This invention relates to apparatus and method whereby light intensity modulating fiber optic sensors may be self-referenced to reduce variable losses due to the optical leads, connectors, and other optical components. Thus, the output signal from such apparatus will vary only with the detected phenomenon and not transmission losses. This invention also relates to a self-referencing system in which a multiplexing technique is used to provide clear output signals from a plurality of fiber optic sensors coupled in one system.
The explosion in fiber optic technology is transforming many areas of technical endeavor. For example, the telecommunications industry is undergoing an infusion of fiber optic devices and expertise. The intensive research and development efforts directed to fiber optic technology have produced a wide array of potential applications for fiber optic devices. Paramount among these potential uses is fiber optic sensing.
Over the past few years, fiber optic sensing systems have been proposed for a wide variety of uses such as temperature sensing, pressure sensing, acceleration measurement, measurement of the flow of liquids and gases, strain sensing, the measurement of a liquid level in a container, etc. The rapid increase in optical fiber sensors is attributable to their marked advantages over known sensing systems. For example, it is known that optical fiber sensors are more rugged and more resistant to corrosion than other sensors. Also, fiber optic sensors are relatively immune to electromagnetic interference. In addition, fiber optic sensing systems appear to be more sensitive, less expensive and more reliable than known sensing devices. The current state of optical fiber sensors is described in the article "Optical-Fiber Sensors Challenge The Competition", by Giallorenzi et al, appearing in IEEE Spectrum, September 1986.
However, the wide-spread use of such optical sensors has been somewhat inhibited due to the signal-to-noise ratio problems inherent in such systems. Specifically, transmission losses such as optical lead losses, connector losses, and other optical component losses may actually exceed the useable signal provided at the output of the system. For example, an optical pressure transducer may provide an output signal which is within the noise level of the optical system. Therefore, until an appropriate solution is found for overcoming such transmission losses, the use of fiber optic sensing systems may be somewhat delayed.
An additional problem with known fiber optic sensors is that the drift in the light source may also overshadow the optical transducer output. This problem is described briefly in the IEEE Article discussed above. Thus, successful fiber optic sensing technology requires a solution to the problems of transmission losses and optical source drift.